Two decades of X-ray observations of the isolated neutron star RX J1856.5-3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement

The soft X-ray pulsar RX J1856.5-3754 is the brightest member of a small class of thermally-emitting, radio-silent, isolated neutron stars. Its X-ray spectrum is almost indistinguishable from a blackbody with $k{T}^{\mathrm{\infty }}\approx 60$ eV, but evidence of harder emission above $\sim 1$ keV has been recently found. We report on a spectral and timing analysis of RX J1856.5-3754 based on the large amount of data collected by XMM-Newton in 2002–2022, complemented by a dense monitoring campaign carried out by NICER in 2019. Through a phase-coherent timing analysis we obtained an improved value of the spin-down rate $\dot{\nu }=-6.042(4)\times {10}^{-16}$ Hz s${}^{-1}$, reducing by more than one order magnitude the uncertainty of the previous measurement, and yielding a characteristic spin-down field of $1.47\times {10}^{13}$ G. We also detect two spectral components above $\sim 1$ keV: a blackbody-like one with $k{T}^{\mathrm{\infty }}=138\pm 13$ eV and emitting radius $31_{-16}^{+8}$ m, and a power law with photon index $\mathrm{\Gamma }=1.4_{-0.4}^{+0.5}$. The power-law 2–8,keV flux, $(2.5_{-0.6}^{+0.7})\times 10-15$ erg cm${}^{-2}$ s${}^{-1}$, corresponds to an efficiency of ${10}^{-3}$, in line with that seen in other pulsars. We also reveal a small difference between the $0.1$–$0.3$ keV and $0.3$–$1.2$ keV pulse profiles, as well as some evidence for a modulation above $1.2$ keV. These results show that, notwithstanding its simple spectrum, \eighteen still has a non-trivial thermal surface distribution and features non-thermal emission as seen in other pulsars with higher spin-down power.